Cost-Effective Reciprocating Engine Emissions Control and Monitoring
for E&P Field and Gathering Engines

Last Reviewed 12/30/2011

DE-FC26-02NT15464

Project Goal
This project is intended to create a computerized database inventory of compressor engines used in the oil and natural gas exploration and production (E&P) industry. This database will be used to evaluate emissions control technologies in both the lab and field to encourage technology transfer.

Performer
Kansas State University, Manhattan, KS

Background
The overall objective of the proposed work is to identify, develop, test, and commercialize emissions control and monitoring technologies that can be implemented by E&P companies to significantly lower their cost of environmental compliance and expedite project permitting. The worst-case scenario is that the existing fleet of engines would have to be replaced to comply with emission regulations. This proposal focuses on retrofit technologies that can be installed on the existing fleet for a fraction of the cost of installing new engines.

The project team is taking considerable advantage of the research and development efforts and practices that have been underway in the gas pipeline industry for the last 12 years. It is interfacing closely with the E&P industry to develop cost-effective options that apply to widely used field and gathering engines and that can be readily commercialized.

Impact
The testing and evaluation database on engine emissions will provide the petroleum industry with a useful tool for selecting the best engine emission reduction strategies for specific field implementation. Technology transfer will be focused through an E&P company liaison group. The goal is to engineer retrofit systems that are significantly less expensive than replacing engines. The cost savings to the U.S. consumer could easily reach millions of dollars.

Accomplishments

A demographic analysis of the sizes and types of engines that power the United States’ natural gas gathering infrastructure was developed. Armed with this information, the researchers conducted an aggressive battery of tests to analyze cost–effective nitrogen oxide (NOx) and carbon monoxide (CO) emissions reduction technologies that can be installed on these engines.

Later research included conducting laboratory tests to determine prototype emissions controls and full-scale field testing of these controls on representative field engine models with challenging emissions profiles. The project assessed emission control technologies applied to various engines typically used in E&P applications, including two-stroke cycle lean-burn engines and four-stroke cycle, lean- or rich-burn engines. A one-cylinder Ajax DP-115 (13½-inch bore by 16-inch stroke) was used to confirm viability for a variety of promising low-emission technologies for lean-burn engines, while various field engines were used to confirm the effectiveness of technologies for rich-burn engines. These technologies are targeted toward better-enabling E&P oil and gas field engines to meet clean air requirements.

The first series of tests varied the air-to-fuel ratio and ignition timing to determine the optimal fueling rate, efficiency, and emissions from the baseline engine. The tests indicated that increasing air flow to the combustion chamber while retarding the ignition timing reduces NOx emissions and is particularly useful when combined with a precombustion chamber. Subsequent tests investigated the impact of the precombustion chamber design, ignition, and fuel injection on emissions reduction, and the ability of ion sensors and fiber optic pressure sensors to measure in-cylinder parameters in real time. The battery of tests shows which technologies have the capability to optimize engine performance and reduce NO. These tests were completed in an efficient and cost-effective manner by using the Ajax DP-115 as a surrogate for the smaller E&P engines, The various technologies can be swapped quickly by exchanging hardware, and the cost to operate the engine is low.

Initial work on rich-burn technologies indicated non-selective catalytic reduction (NSCR) would be the final solution. However, reports disseminated within the natural gas industry in the summer of 2006 showed existing NSCR was less effective than previously thought. Additionally, impending regulations in Colorado and New Mexico will regulate even the smallest engines, for which NSCR has not yet been effectively demonstrated. Thus, it was determined that additional characterization of issues that impede reliability of this technology was needed before beginning full-scale field testing. This characterization was performed on field engines in the Four Corners region. Approximately $500,000 from cost share participants was identified—data collection began in September 2007 and continued through August 2008. The field testing of lean burn engines took place during the summer and fall of 2008.

Dr. Kirby Chapman and researchers co-authored two presentations for the 2009 Spring Internal Combustion Engine Division Technical Conference. The meeting was sponsored by the American Society of Mechanical Engineers (ASME). The presentation entitled "Variation in Long-Term Emissions Data from NSCR-Equipped Natural Gas-Fueled Engines" discussed the research carried out to characterize pollutant emissions performance of non-selective catalytic reduction (NSCR) technology on three engines in the Four Corners area. Data shows significant variation in emissions levels over time, as well as seasonal variation. As a result of these variations, simultaneous control of NOx to below a few hundred parts per million (ppm) and CO to below 1,000 ppm volumetric concentration was not consistently achieved. Instead, the NSCR/AFRC systems were able to simultaneously control both species to these levels for only a fraction of the time the engines were monitored. Both semi-continuous emissions data and periodically collected emissions data support a NOx-CO trade-off and a NOx-ammonia tradeoff in NSCR-equipped engines.

The second presentation entitled "Mapping Study to Characterize NSCR Performance on a Natural Gas-Fueled Engine" focused on the characterization of pollutant emissions performance of field gas-fired four-stroke cycle rich burn (4SCRB) engines equipped with non-selective catalytic reduction (NSCR) technology. In the mapping study, ammonia, formaldehyde, CO, NOx, and speciated hydrocarbon emissions were recorded in real-time using an extractive FTIR system. The mapping tests demonstrated a trade-off between NOx emissions and CO, ammonia, and hydrocarbon emissions. Richer engine operation decreases NOx emissions at the expense of higher CO, ammonia, and hydrocarbon emissions. Leaner operation has the opposite effect.

In both the semi-continuous monitoring portion and the mapping portion of this program, currently-available NSCR/AFRC systems were shown to provide NOx and CO emissions control. The data indicate a fairly tight operating window for simultaneous control of both NOx and CO to low levels. The study supports previous work indicating that post-catalyst emissions depend on pre-catalyst oxygen concentration, which also is used to indicate the air-to-fuel ratio for this project. CO emissions increase and NOx emissions decrease as engine operation becomes richer. Conversely, NOx increases as pre-catalyst oxygen increases, and CO decreases as precatalyst oxygen increases. NOx emission control is very effective until the pre-catalyst oxygen concentration surpasses a certain concentration, after which NOx emission increases rapidly. The result is that a trade-off relationship exists not only between NOx and CO, but also between NOx and NH3 and between NOx and THC. The tested catalysts were quite effective in removing formaldehyde across the entire oxygen range of the effective catalyst window.

Effort was focused on the theoretical and analytical work required to improve the performance of currently available NSCR/AFRC systems. The objective of this task was to conduct the much-needed analytical, theoretical, and experimental work to determine to what degree various factors affect the emissions stability of the tested systems as well as other systems. This task continues to mine the field test data to harvest knowledge that can then be used to develop a next-generation NSCR/AFRC system. Due to the inconsistency of the EGO sensor output and its impact on the overall NSCR/AFRC systems capabilities, the research team will focus on developing a parametric, physics-based model of the EGO sensor. The research team began this task by conducting a comprehensive literature review of EGO sensors. This literature review provides background on the EGO sensor working principles and describes the prior work done to model EGO sensors.

A literature review and engineering approach provided information regarding the development of a 4SC engine model and an EGO sensor model. The 4SC engine model employs a quasi-geometric three zone model of unburned and burned gasses. Both NOx and CO are described via detailed chemical kinetics processes by which these pollutants are formed. The engineering approach for modeling the EGO sensor was developed. The model was divided into three different modules. The protective layer module is responsible for modeling the diffusion of each exhaust gas species through the sensor protective layer. The main governing equations used in this module are the mass conservation equation of each constituent in conjunction with the Maxwell-Stefan equation which is used for calculating the diffusion fluxes. The concentration of each exhaust gas species can be determined by applying the governing equations to each of these finite control volumes.

A kinetic model was created from experimental data to describe methane combustion, but the complex combustion mechanism could not be fully developed. Therefore, a cycle-resolved correlation for CO formation and oxidation in 4SC engines will be defined from engine field data. The new kinetic algorithm for CO formation takes into account the changing temperatures and pressures in the cylinder.

The research team has recently focused its efforts on detailing the EGO sensor model which produces output very comparable to experimental data. The developed model includes the transport of exhaust gas species through the sensor protective layer, the detailed surface catalytic reactions on the sensor electrode, and the electrochemistry of the electrolyte material. The model confirms that the sensor output depends not only on the oxygen concentration, but also on the other exhaust gas reducing species such as CO and H2.

Researchers made a presentation at the Gas Machinery Conference that was held in Phoenix, AZ, October 4–6, 2010.

Researchers continued to fine tune the 4-stroke cycle engine model by developing a methodology for quantifying unburned hydrocarbons based on engine geometry. Modeled results matched previous field test data. The reaction scheme for the EGO sensor model has been updated and is less complex and requires less computing time than the previous version. A “lean shift” has been detected when methane present in the exhaust emissions generates a higher output voltage from the sensor. This is due to the extra reducing species present that compete with oxygen for the catalytic surfaces.

Dr. Kirby Chapman collaborated with Jim McCarthy from Innovative Environmental Solutions to determine the possible impacts of the new EPA National Ambient Air Quality Standards (NAAQS) and NO2 levels on small engine emissions. The compliance measures are based on plume models which take into consideration stack height, flow, and concentrations. These models were developed for larger sources and not smaller engines so a new model may need to be developed. Dr. Chapman is developing a Four Stroke Cycle Engine Model and an EGO Sensor Model. Both models will consider the most important species to keep operation simple and less cumbersome.

Researchers used field data from natural gas fueled engines to test the EGO Sensor Model. Model results agreed with the field data and showed that the sensor output voltage is dependent on CO and H2 levels in the exhaust. Methane levels have a less significant effect on sensor output. The engine mapping data is well explained with the modeled results.

Field testing of a 4-stroke rich-burn engine was performed with NSCR catalysts, various air-to-fuel controllers and a modified UEGO sensor. The UEGO has been developed to better model conditions of natural gas exhausts. With this added UEGO sensor, the air-to-fuel controllers were better able to maintain emission compliance.

Current Status (October 2011)
The project ended August 31, 2011 and the final report is available below under "Additional Information".